In clinical studies arachnoiditis has been attributed most commonly to myelographic contrast media, or spinal surgery. Experimental studies have verified that aqueous or oily myelographic contrast media may produce arachnoiditis. In some cases of arachnoiditis, especially focal root sheath scarring, the cause has not been clarified. We propose to examine the role of two potential contributing factors to the devolpment of focal arachnoiditis: anatomic distortion and compression of a nerve root sheath and local exposure of a nerve root sheath to some components of nucleus pulposus. Nerve rot compression will be produced by operative placement of small bone fragements in an appropriate location to stretch the nerve root. Exposure to components of intervertebral disc material will be produced by injection of washed polytroned monkey nucleus pulposus or specific components of interevertebral disc into the epidural space surrounding a nerve root. These animals will be studied at 12 weeks with myelography to determine the extent of arachnoiditis. They will then be sacrificed and the lumbar arachnoid examined and scored by light microscopy using previously published criteria for arachnoiditis. Following studies in previously normal animals, the effect of spinal surgery and contrast myelography on the arachnoiditis caused by nerve compression or exposure to disc material will be examined.